1. Field of the Invention
The invention relates to detection and monitoring devices and is directed more particularly to an assembly for detecting and monitoring the presence of scour in underwater beds, such as river beds, navigational channels, and the like.
2. Description of the Prior Art
Scour is a severe problem that results in millions of dollars of damage to infrastructure and substantial loss of life annually. Scour occurs during times of high tides, hurricanes, rapid river flow, and icing conditions when sediment, including rocks, gravel, sand, and silt, are transported by currents, undermining bridge pier foundations, submarine utility cables and pipelines, and filling in navigational channels. Scour is dynamic; ablation and deposition can occur during the same high energy hydrodynamic event. The net effect cannot be easily predicted nor readily monitored in real-time.
Bridge scour monitoring technologies are known. In U.S. Pat. No. 5,784,338, issued Jul. 21, 1998 to Norbert E. Yankielun et al, an instrument called a "time domain reflectomer" (TDR) is directly connected to a parallel transmission line consisting of a pair of robust, specially fabricated non-corroding rods or wires. The principle of TDR is known, described in the technical literature, and applied to numerous measurement and testing applications. This technique was applied to scour detection and monitoring in the '338 patent, which is incorporated herein by reference. TDR operates by generating an electromagnetic pulse, or a fast rise time step, and coupling it to a transmission line. The pulse propagates down the transmission line at a fixed and calculable velocity, a function of the speed of light and the electrical and physical characteristics of the transmission line. The pulse propagates down the transmission line until the end of the line is reached, and is then reflected back toward the source. The time in seconds that it takes for the pulse to propagate down and back the length of the transmission line is called the "round trip travel time" and is calculated as described in the '338 patent.
For a two wire parallel transmission line, changes in the dielectric media in the immediate surrounding volume causes a change in the round trip travel time. Freshwater has a relatively high dielectric constant and dry sedimentary materials (e.g.: soil, gravel and stone) have a relatively low dielectric constant. Wet sediment has a dielectric constant that is a mixture of those of water and dry soil. The dielectric constant of this mixture will vary depending upon the local sedimentary material constituency, but in all cases of bulk dielectric (bulk index of refraction) of the mixture will be less than that of liquid water alone and significantly greater than that of the dry sedimentary materials. Some sediment materials, particularly clay-based sediments, can be extremely "lossy". This lossy behavior of the soil is exhibited by a severe attenuation of an electromagnetic pulse as it propagates along a transmission line surrounded by such materials. The pulse, when launched from a TDR will dissipate as it travels along the transmission line. Sufficient dissipation will reduce the reflected pulse energy below a detectable level.
At any boundary condition along the transmission line (e.g., air/water and water/sediment), a dielectric discontinuity exists. As a pulse traveling down the transmission line from the TDR source encounters these boundary conditions, a portion of pulse energy is reflected back to the source from the boundary. A portion of the energy continues to propagate through the boundary until another boundary or the end of the cable causes all or part of the remaining pulse energy to return along the transmission line toward the source. Measuring the time of flight of the pulse and knowing the dielectric medium through which the pulse is traveling permits calculation of the physical distance from the TDR source of each of the dielectric interface boundaries encountered.
For lossy consolidated soils, such as clay, the electromagnetic signal is greatly attenuated as it propagates along the imbedded transmission line. Levels of signal attenuation can be as much as 10's of decibels per meter. This results in little or no reflected signal returned to the instrument over the length of the probe buried in the lossy media. If the sensor source is imbedded in lossy media along with a portion of the sensor probe, the media will absorb (dissipate) all the pulse energy. Little or no reflected signal is returned. If a pulse is propagating along a transmission line imbedded in a non- or minimally-lossy material and a boundary with some extremely lossy material is encountered, a reflection will occur at the interface boundary, similarly to that that would occur for a boundary between two non-lossy materials. The magnitude of the reflection will be proportional to the reflection coefficient of the two materials at the interface.
In U.S. Pat. No. 5,790,471, issued Aug. 4, 1998 to Norbert E. Yankielun et al, there is disclosed a Water/Sediment Interface Monitoring System Using a Frequency-Modulated Continuous Wave reflectometer. The frequency modulated continuous wave (FM-CW) technique is known with respect to radar systems. In this system, instead of launching electromagnetic waves from an antenna into free space, as would be done in a radar application, the waves are coupled to a transmission line, such as a parallel line sensor or the like, as described above.
In an FM-CW reflectometer system, a steady amplitude signal whose frequency increases linearly with time is transmitted down a transmission line. The FM-CW signal is produced by a voltage controlled oscillator (VCO) driven by a linear ramp generator. This signal, coupled to the transmission line, propagates down the line and is reflected from the far end, or intermediate discontinuity, returning to the source, delayed by the round-trip propagation time, all as described in the '471 patent which is incorporated herein by reference.
This signal can be directly processed, analyzed, and stored or displayed. Alternatively, this signal can be transmitted to a remote location over twisted pair, coaxial cable, radio, or other form of telemetry, where it then can be processed, analyzed, etc.
Lossy sediments cause the same signal dissipation effect with the FM-CW reflectometer as experienced with the TDR. These instruments have proved successful in detecting, monitoring and measuring scour and deposition of sediments in freshwater. Existing implementations use a transmission sensor consisting of two parallel rods. This two-pronged unit is physically large and robust, and can survive deployment in high current rivers. This implementation, while successful at measuring sedimentation and scour, has a large cross-section, and therefore a tendency to more easily snag submerged current-borne debris. The implementation disclosed here employs a single, robust, vertical cylinder with sensor transmission line wires (or conductive tapes) bonded vertically along the outside axial length of the cylinder. The entire unit is watertight and corrosion-resistant. The cylinder houses the scour probe electronics and other associated hardware. This probe is suitable for use in both high current and low flow in freshwater applications and will operate in low-loss or lossy sediments.
There is a need for a probe for detecting and monitoring scour, which probe is structured to accept either TDR of FM-CW reflectometer packages and which is provided with a limited profile, or cross-section so as to reduce collision with water-borne debris.